This invention relates to a cryopump and more particularly to a novel method and apparatus for removing gases from a high vacuum environment.
The semiconductor and metallurgical industries among others require a vacuum facility that can be operated at a relatively high vacuum, in the range of 10.sup.-4 to 10.sup.-6 Torr (where a Torr is 1/760th of atmospheric pressure). Cryopumps are frequently used to remove gases from work spaces, because they operate effectively at high vacuum when external heat losses are minimized.
Cryopumps use a refrigerative process to enhance and maintain high vacuum environments. Sub-atmospheric gases with relatively high vaporization temperatures are passed over a cold surface in the range of 77 to 20 degrees Kelvin, resulting in such gases being deposited on the cold surface in the form of fluid or as a solid, commonly referred to as "ice". The cold surface, called a "cryopanel", thus removes (i.e. "pumps") gases from the environment by the process of condensation, termed "cryocondensation", and the process of adsorption, termed "cryosorption".
In the prior art, the effectiveness of the cryopump is diminished by the accumulation of ice on the cryopanel. During the first stages of accumulation, the pumping action simply slows, removing less gas per unit of time. Eventually accumulation of ice progresses to a point where gas removal is slowed to a degree that system pressure rises above a desired level. Further system operation requires that the system be shut down and "regenerated" by allowing the cryopanel to return to a higher temperature so that solidified gases may sublimate, be released, and pumped from the system by other means.
Continuous cryopumping (in the sense that the high vacuum remains unbroken) is accomplished in most cases by operating batch type cryopumps, alternately regenerating first one and then the other. Much of the prior art is thus devoted to increasing the cryopumping system's effectiveness by minimizing the accumulation of solidified gas.
One way to delay regeneration is to over-design the cryopump. Given two cryopumps of different sizes, removing the same gas load, the smaller pump will need regeneration before the larger pump. Common practice, therefore, is to build large cryopumps with large adsorption capacities.
The throttle valve method of the prior art is also based upon the cryopump being over-designed for a given application. Gas flow to the cryopump surface is restricted, which in effect is equivalent to reducing pumping speed and, therefore, extending time between regenerations. Continuous long term cryopumping is not a feature. The double stage pumping method of the prior art emphasizes differential pumping of water vapor and inert gases, as distinct from continuous long-term high through-put. However, high speed pumping of low condensing temperature gases is severely restricted by using staged pumping. (For a discussion of these methods, see U.S. Pat. No. 4,449,373, issued May 22, 1984, and U.S. Pat. No. 4,475,349, issued Oct. 9, 1984.)
A more recent method in the prior art uses a cylindrical chamber and a scraper moving in a helical motion around and up the walls of the inside of the chamber. The scraper chips ice from the surface and exhausts it as a solid or gas. While this method accomplishes cryopumping without the need for shutdown for regeneration, pressure fluctuations result from the reciprocal motion of the scraper and the geometry of the cryopanel. (U.S. Pat. No. 4,724,677 issued Feb. 16, 1988. See further, Foster, "High-Throughout Continuous Cryopump," J. Vac. Sci. Technol. A5(4), July/August 1987.)
It is an object of applicants' invention to provide an improved cryopumping system for removing solidified gases from a high vacuum environment without the need for a shutdown to accomplish regeneration.
It is another object of this invention to provide a cryopumping system from which solidified gases are removed at an average rate substantially equal to the rate at which they are deposited so that vacuum pressure is substantially constant.
It is another object of this invention to provide an improved cryopumping system that minimizes the heat transfer from the cryopanel to the refrigerant flow.
It is another object of this invention to provide a method for reducing the required physical dimensions of the cryopanel.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.